docs: def true_num_reactions(model, custom_spont_id=None):
      """Return the number of reactions associated with a gene.
  
      Args:
          model (Model):
          custom_spont_id (str): Optional custom spontaneous ID if it does not match the regular expression ``[Ss](_|)0001``
  
      Returns:
          int: Number of reactions associated with a gene
  
      """
      true_num = 0
      for rxn in model.reactions:
          if len(rxn.genes) == 0:
              continue
          if len(rxn.genes) == 1 and is_spontaneous(list(rxn.genes)[0], custom_id=custom_spont_id):
              continue
          else:
              true_num += 1
      return true_num